This application is based upon and claims benefit of priority of Japanese Patent Applications No. Hei-10-70698 filed on Mar. 19, 1998, and No. Hei-10-300031 filed on Oct. 21, 1998, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electroluminescent display device having a matrix electrode structure, and more particularly to such an electroluminescent display device in which an uneven luminance caused by an electrode resistance is substantially eliminated.
2. Description of Related Art
Generally, an electroluminescent display device (hereinafter referred to as an EL display device) includes an EL panel having scanning electrodes and data electrodes, both electrodes being arranged to form a matrix, a luminescent layer disposed between both electrodes, and driving circuits for driving both electrodes. Pixels are formed at each intersection of both electrodes. The scanning electrodes are sequentially scanned, and at the same time modulation voltages are imposed on the data electrodes. Images are displayed on the EL panel by the pixels arranged in a form of a matrix.
The scanning electrodes are usually made of a metallic material and the data electrodes are made of a transparent material such as ITO. Since the transparent material such as ITO has an electric resistance which is more than ten times (e.g., 10-1000 times) higher than that of a metallic material such as aluminum or chromium, uniformity of display luminance is adversely affected by the electric resistance, and an uneven luminance appears on the display. In order to cope with this problem, a display device disclosed in U.S. Pat. No. 5,311,169 employs a special control of data voltages. That is, a pulse width of the modulation voltage imposed on the data electrodes is increased or decreased according to a scanning sequence of the scanning electrodes. However, this control method is complex and requires an expensive control device. Display devices using a transparent material for both scanning and data electrodes are also proposed. However, no effective solution against the uneven luminance in such a device has been proposed. The uneven luminance problem is especially notable when images are displayed with a lower luminance by decreasing the pulse width of scanning or data voltages.
There are some EL devices in which the scanning electrodes are made of the transparent material such as ITO. In the device of this kind, a scanning voltage pulse is gradually deformed as a distance from an end from which the scanning voltages are imposed increases because of the electric resistance of the transparent electrodes. As a result, the luminance of the display becomes uneven across the display panel. To solve this problem, EP 344323 proposes to impose the scanning voltages from both ends of the scanning electrodes at the same time. Two separate scanning electrode driving circuits are disposed at both sides of the scanning electrodes. However, an excessive current may flow through the scanning electrodes if phases of the pulse voltages imposed from both sides are different from each other. That is, if rising and falling edges of the pulse voltages imposed from both sides do not appear at the same time, two driving circuits are short-circuited.
The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide an EL display device having transparent electrodes that have a relatively high electric resistance, in which images are displayed with a high luminance uniformity, and more particularly to realize such a display device with a less complex control.
An electroluminescent (EL) display device is composed of a display panel and driving circuits for driving the display panel. In the display panel, an electroluminescent layer is sandwiched between scanning electrodes and data electrodes, both electrodes forming a matrix structure. The driving circuits include a first and a second scanning electrode driving circuit and a data electrode driving circuit. The first scanning electrode driving circuit is connected to one side of the scanning electrodes, while the second scanning electrode driving circuit is connected to the other side of the scanning electrodes. Scanning pulse voltages are supplied to the scanning electrodes alternately from both sides, so that when the first scanning electrode driving circuit supplies the scanning voltage, the second circuit does not operate, and vice versa.
Generally, the luminance of the EL panel is higher at a side where the driving voltage is supplied and lower at the other side because of electric resistance of the scanning electrodes. According to the present invention, the scanning voltages are supplied alternately from both sides, and an overall luminance of the EL panel is a combined luminance obtained by scanning the scanning electrodes from both sides. Therefore, the overall luminance becomes substantially uniform on the surface of the EL panel.
Alternatively, the scanning voltages may be supplied simultaneously from both sides of the scanning electrodes. In this case, the phase of the scanning voltages supplied from both sides has to be equalized. If the scanning voltages are supplied from both sides at different timing or with different phases, excessive current flows through the scanning electrodes, and the first and second scanning electrode driving circuits would be short-circuited in the worst case. According to the present invention, output stages of the scanning electrode driving circuits are brought into a high impedance state for predetermined periods before and after the scanning voltage is supplied, in order to surely avoid such an excessive current or short-circuit. The uniformity of the luminance is attained by supplying the scanning voltages simultaneously from both sides.
The data electrodes may be driven from both sides thereof either alternately or simultaneously, if it is required to eliminate uneven luminance along the data electrodes. Further, the scanning electrode driving circuits connected to both sides of the scanning electrodes may be unified into a single driving circuit which supplies the scanning voltages from both sides of the scanning electrodes. To drive the electrodes alternately from both sides by the single driving circuit, its outputs are alternately switched from one side to the other side. This invention may be applied also to a liquid crystal display device having a matrix electrode structure.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings.